The present invention relates to impingement cooling of a gas turbine nozzle band side wall of a nozzle segment and particularly relates to impingement cooling of a nozzle band side wall in the undercut region of a nozzle segment wherein the weld joint between the nozzle segment cover and the nozzle side wall is remote from the nozzle wall exposed to the hot gas path.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner annular bands and a plurality of vanes extend between the bands. The bands and vanes define in part the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more nozzle vanes extend between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments to cool the parts exposed to the hot gas path. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. The impingement plate defines with the cover a first cavity on one side thereof for receiving cooling steam from a cooling steam inlet. The impingement plate also defines, along an opposite side thereof and with the nozzle wall, a second cavity. The impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the nozzle wall. The cooling steam then flows radially inwardly through cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. The cooling steam then enters a chamber in the inner band portion and reverses its flow direction for flow radially outwardly through an impingement plate for impingement cooling the nozzle wall of the inner band. The spent cooling medium flows back through a cavity in the vane to an exhaust port of the nozzle segment.
The cover provided in each of the outer and inner band portions is preferably welded to the corresponding nozzle side wall. In prior designs, the weld joint between the cover and the nozzle side wall was disposed at a radial location between the nozzle wall and the spline seal between side walls of adjacent nozzle segments. In that location, the weld was exposed to the high temperature gases in the hot gas flow path and was very difficult to cool. Thus, weld joint fatigue life was significantly reduced due to its proximity to the hot gas path. Moreover, the location of the weld was not optimum for manufacturing repeatability and was very sensitive to manufacturing tolerances. The weld joint was characterized by variable wall thicknesses which increased the stress at the joint, decreased the low cycle fatigue and limited the life of the parts. The wall thickness at the weld after machining was also a variable which could not be tolerated in the manufacturing process.
In accordance with a preferred embodiment of the present invention, a cooling system is provided in a nozzle segment in which the weld joint between the cover and nozzle wall is on the side of the spline seal remote from the nozzle wall exposed to the hot gas path. That is, the weld joint between the cover and the nozzle side wall of the outer band is located radially outwardly of the spline seal between adjacent outer bands while the weld joint between the cover and the nozzle side wall of the inner band is located radially inwardly of the spline seal between adjacent inner bands. This reduces the temperature of the weld joints during turbine operation, reduces the stresses across the joints, both thermal and mechanical, eliminates any requirement for machining after welding and results in joints of constant thickness and higher fatigue life. The location also leads to improved machinability and tolerance to weld defects.
To provide that weld location, undercut regions adjacent the side walls of the nozzle segment bands are formed. Particularly, each undercut region includes a side wall or edge of the nozzle segment and an inturned flange extending inwardly from and generally parallel to and spaced from the nozzle wall. Cooling the nozzle band side wall or edge, however, is quite difficult in view of the undercut region which distances the side wall or edge from the impingement plate. This large distance reduces the effectiveness of cooling the nozzle side wall by impingement cooling flow through apertures in the impingement plate.
In accordance with the present invention, improved side wall fabrication and cooling is provided. Particularly, with the weld joint between the cover and the nozzle side wall located remotely from the hot gas path through the turbine, side wall cooling is improved by providing a backing plate for the impingement plate with apertures through the backing plate aligned with apertures through the impingement plate for directing impingement cooling flow onto the side wall. Particularly, the impingement plate is provided with a turned edge. Margins of the edge are secured, for example, by welding to the prepared face of the inturned flange of the nozzle segment side wall, leaving a portion of the turned edge of the impingement plate extending generally parallel to the nozzle segment side wall. To more directly target or focus the impingement cooling medium flowing through the apertures of the turned edge, a backing plate having apertures aligned with the apertures through the turned edge of the impingement plate is secured along the turned edge. As a consequence, the length-to-diameter ratio of the aligned apertures is improved, thereby enabling direct targeting or focusing of the cooling flow onto the side wall of the nozzle segment. The backing plate also adds additional strength about the perimeter of the impingement plate.
The foregoing cooling system is readily and easily fabricated. For example, the backing plate is added to the turned flange of the impingement plate and apertures are then provided simultaneously through the backing plate and turned edge. The impingement plate is then placed into the nozzle segment and tacked into position and later welded or brazed into the nozzle segment.
In a preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions having a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therebetween and an impingement plate secured within the segment and disposed in the chamber defining with the cover a first cavity on one side thereof for receiving a cooling medium, the impingement plate on an opposite side thereof defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures therethrough for flowing cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, the nozzle segment including a side wall extending generally radially between the nozzle wall and the cover and having an inturned flange, the inturned flange defining an undercut region adjacent the side wall, and a backing plate overlying a portion of the impingement plate, the backing plate and the impingement plate portion having aligned apertures therethrough for directing a flow of the cooling medium onto the side wall for impingement cooling thereof.